Interferons (IFNs) are a part of the group of intercellular messenger proteins known as cytokines. .alpha.-IFN is the product of a multigene family of at least 16 members, whereas .beta.-IFN is the product of a single gene. .alpha.- and .beta.-IFNs are also known as type I IFNs. Type I IFNs are produced in a variety of cells types. Biosynthesis of type I IFNs is stimulated by viruses and other pathogens, and by various cytokines and growth factors. .gamma.-IFN, also known as type II IFN, is produced in T-cells and natural killer cells. Biosynthesis of type II IFN is stimulated by antigens to which the organism has been sensitized. Both .alpha.- and .DELTA.-IFNs are immunomodulators and anti-inflammatory agents, activating macrophages, T-cells and natural killer cells.
IFNs are part of the body's natural defense to viruses and tumors. They exert these defenses by affecting the function of the immune system and by direct action on pathogens and tumor cells. IFNs mediate these multiple effects in part by inducing the synthesis of many cellular proteins. Some interferon-inducible (IFI) genes are induced equally well by .alpha.-, .beta.- and .gamma.-IFNs. Other IFI genes are preferentially induced by the type I or by the type II IFNs.
The various proteins produced by IFI genes possess antitumor, antiviral and immunomodulatory functions. The expression of tumor antigens in cancer cells is increased by .alpha.-IFN, and renders the cancer cells more susceptible to immune rejection. The IFI proteins synthesized in response to viral infections are known to inhibit viral functions such as cell penetration, uncoating, RNA and protein synthesis, assembly and release (cf Hardman J G et al (1996) The Pharmacological Basis of Therapeutics, McGraw-Hill, New York N.Y. pp 1211-1214). Type II IFN stimulates expression of major histocompatibility complex (MHC) proteins and is thus used in immune response enhancement.
An IFI gene known as 6-16 encodes an mRNA which is highly induced by type I IFNs in a variety of human cells (Kelly J M et al (1986) EMBO J 5:1601-1606). After induction, 6-16 mRNA constitutes as much as 0.1% of the total cellular mRNA. The 6-16 mRNA is present at only very low levels in the absence of type I IFN, and is only weakly induced by type II IFN.
The 6-16 mRNA encodes a hydrophobic protein of 130 amino acids. The first 20 to 23 amino acids comprise a putative signal peptide. Protein 6-16 has at least two predicted transmembrane regions culminating in a negatively charged C-terminus.
The p27 gene encodes a protein with 41% amino acid sequence identity to the 6-16 protein. The p27 gene is expressed in some breast tumor cell lines and in a gastric cancer cell line. In other breast tumor cell lines, in the HeLa cervical cancer cell line, and in fetal lung fibroblasts, p27 expression occurs only upon .alpha.-IFN induction. In one breast tumor cell line, p27 is independently induced by estradiol and by IFN (Rasmussen U B et al (1993) Cancer Res 53:4096-4101).
Expression of p27 was analyzed in 21 primary invasive breast carcinomas, 1 breast cancer bone metastasis, and 3 breast fibroadenomas. High levels of p27 were found in about one-half of the primary carcinomas and in the bone metastasis, but not in the fibroadenomas. These observations suggest that certain breast tumors may produce high levels of, or have increased sensitivity to, type I IFN as compared to other breast tumors (Rasmussen U B et al, supra). In addition, the p27 gene is expressed at significant levels in normal tissues including colon, stomach and lung, but not expressed in placenta, kidney, liver or skin. (Rasmussen U B et al, supra).
The small hydrophobic IFI gene products may contribute to viral resistance. A hepatitis-C virus (HCV)-induced gene, 130-51, was isolated from a cDNA library prepared from chimpanzee liver during the acute phase of the infection. The protein product of this gene has 97% identity to the human 6-16 protein (Kato T et al (1992) Virology 190:856-860). The investigators suggest that HCV infection actively induces IFN expression, which in turn induces expression of IFI genes including 130-51.
The IFI proteins synthesized in response to viral infections are known to inhibit viral functions such as penetration, uncoating, RNA or protein synthesis, assembly or release. The 130-51 protein may inhibit one or more of these functions in HCV. A particular virus may be inhibited in multiple functions by IFI proteins. In addition, the principle inhibitory effect exerted by IFI proteins differs among the virus families (Hardman J G, supra, p 1211).
The hydrophobic IFI proteins of the invention may provide the basis for clinical diagnosis of diseases associated with their induction. These proteins may be useful in the diagnosis and treatment of tumors, viral infections, inflammation, or conditions associated with impaired immunity. Furthermore, these proteins may be used for investigations of the control of gene expression by IFNs and other cytokines in normal and diseased cells.